In biology, animal populations are studied. Particularly, branches of ecology known as population biology and population genetics. In population dynamics, size, age, sex structure, mortality, reproductive behaviour, and growth of a population are studied. In biology, an isolated population denotes a breeding group whose members reproduce mostly/solely among eachother, often as a result of physical isolation, despite that biologically they could reproduce with any member of their species. Metapopulation is a group of sub-populations within an area. Individuals of sub-populations are able to traverse uninhabitable space, driving biological dispersal, a key affecting element over populations within the area.

Demography is the study of human populations. Various aspects of human behavior in populations are studied in sociology, economics, and geography. Study of populations is almost always governed by the laws of probability, and the conclusions of the studies may thus not always be applicable to some individuals. This odd factor may be reduced by statistical means, but such a generalization may be too vague to imply anything. Demography is used extensively in marketing, which relates to economic units, such as retailers, to potential customers. For example, Starbucks, a coffee shop company that wants to sell to a younger audience, looks at the demographics of an area to be able to appeal to this younger audience.

The age and gender distribution of a population within a given nation or region is commonly represented by means of a population pyramid. This is a triangular distribution with the portion of the population along the horizontal X-axis and the 5-year age grouping along the vertical Y-axis. Male population is shown to the left of the vertical axis and female to the right.

This type of chart displays the development of a population over a period of time. Nations with low infant mortality and high longevity will display a more rectangular shape as a majority of the population living to old age. The converse will have a more pyramidal shape with a wide base, reflecting higher infant mortality and greater risk of early death.

In biology, a rarely occurring situation in which a group of individuals of a species appear in a new, inhabitable area suitable for more individuals, and begin to populate it. This may also happen if individuals of a species have been transferred to new areas on purpose or by accident. Ecological niches are usually populated, but evolution of a species may enable it to overcome the difficulties encountered in an initially hostile environment.

The world's human population is currently growing by more than 75 million people per year. About half the world lives in nations with sub-replacement fertility, and population growth in those countries is due to immigration. Overpopulation can result from increases in births and survival rates, or from an unsustainable use and depletion of resources. Advances in technology can reduce the threat of overpopulation by making new resources available, or by increasing the productivity of existing resources.

In biology, a classic example of an overpopulation are the lemmings in Lapland, which procreate over the years to such densities, that a great part of the population is forced to wander to inhospitable areas. Nowadays, this happens usually in less dramatic ways than in the past, one reason probably being that the food supply of lemmings is shared with an increased number of reindeer in Lapland.

Population control is the practice of curtailing population increase, usually by reducing the birth rate. Surviving records from Ancient Greece document the first known examples of population control. These include the colonization movement, which saw Greek outposts being built across the Mediterranean and Black Sea basins to accommodate the excess population of individual states. An important example of mandated population control is China's one-child policy, in which having more than one child is made extremely unattractive. This has led to allegations that practices like infanticide, forced abortions, and forced sterilization are used as a result of the policy.

In ecology, population control is on occasions considered to be done solely by predators, diseases, parasites, and environmental factors. At many times human effects on animal and plant populations are also considered. See also [1]. Migrations of animals may be seen as a natural way of population control, for the food on land is more abundant on some seasons. The area of the migrations' start is left to reproduce the food supply for large mass of animals next time around. See also immigration.

Population decline is a fall in a region's population. It can be caused by sub-replacement fertility or heavy emigration, or more dramatically disease, famine, or war. Or most often by a combination of the factors. In the past population decline was mostly observed due to disease. In recent years, the population of Russia and seventeen other ex-Communist countries has begun to decline (1995-2005). The Black Death in Europe, the arrival of Old World diseases to the Americas, or the Irish Potato Famine (1845-1849), all caused massive population declines.

In biology, population decline of a species is usually described as a result of gradually worsening environmental factors, such as prolonged drought or loss of inhabitable areas for the studied species. These, or other factors, may lead to a small population, in which case genetical factors may become dominant in the survival, or extinction of a population.

Population ageing occurs when the fertility rate declines. This means that, for a period of time, the ratio of old to young will be higher than average. It also occurs due to increasing life expectancy. Japan and Western Europe are the two regions which are most confronted by severe population ageing in the near future. The second largest expenditure of most governments is education and these expenses will fall with an ageing population. However older people tend to be the section of the population most concerned about crime and most insistent on more (and more expensive) law and order.

Population transfer is a term referring to a policy by which a state forces the movement of a large group of people out of a region, most frequently on the basis of their ethnicity or religion. This has occurred in India and Pakistan, between Turkey and Greece, and in Eastern Europe after the Second World War. Other movements in population are caused by immigration, such as the immigration from Europe to European colonies in the Americas, Africa, Australia and other places.

A "population bomb," as defined in the book, requires three things: a rapid rate of change; a limit of some sort; and delays in perceiving the limit. The book's specific prediction that "in the 1970s and 1980s hundreds of millions of people will starve to death" did not come to pass, however, due for the most part to the efforts of Norman Borlaug's "Green Revolution" of the 1960s.

It was later shown by Keith Greiner (1994) that Ehrlich's projections could not possibly have held the scrutiny of time, because Ehrlich applied the financial compound interest formula to population growth. Using two sets of assumptions based on Ehrlich's hypothesis, it was shown that the theorized wild growth in population and subsequent scarcity of resources could not have occurred on Ehrlich's time schedule.

According to estimates published by the United States Census Bureau, the world population at the beginning of December 2005 was about 6,483,600,000. The United Nations Population Fund designated October 12, 1999 as the approximate day on which world population reached six billion. This was about 12 years after world population reached five billion, in 1987.

About 4 billion of the world's nearly 6.5 billion people live in Asia. Seven of the world's ten largest countries by population are in Asia (although Russia is also located in Europe). However, a large population is not the same thing as economic size, and the United States, which has a much smaller population than India or China, has a much larger economy.